Inspection systems are known which comprise a remote-controlled automated machine which is capable of moving on the hull of a vessel. To this end, the movable machine is provided with a movement means which comprises an adhesion means which allows the machine to remain in contact with the hull, and a driving means for moving the machine on the surface of the hull of the vessel. The machine also carries on-board various types of sensor in order to carry out local measurements of characteristic physical values of the hull. The machine also comprises a positioning means which allows an instantaneous position to be obtained for the machine relative to a reference point which is preferably associated with the hull of the vessel. The association of a measurement of a physical value and the position of the machine at the time this measurement was carried out allows a map of the hull to be produced by moving the machine along a suitable trajectory.
Up to the present time, as described, for example, in document FR 2 861 457, the positioning means is a means for positioning under water or a means for positioning in the air. Consequently, the inspection system is used, on the submerged portion of the hull of a vessel, that is to say, below the water line of the vessel, or on the non-submerged portion, that is to say, above the water line.
A known means for positioning under water comprises an acoustic transmitter which is arranged on the movable machine, and two acoustic receivers which are submerged and fixed to two buoys floating on the surface of the water, respectively. The buoys, which are remote from each other, are positioned in an absolute manner by a system of the GPS type. The transmitter generates a periodic acoustic signal, and the time correlation of the signals received, at the buoys, by each of the receivers allows the position of the machine to be determined relative to the buoys by means of triangulation. Such a means for positioning under water offers a maximum precision of the order of 50 cm.
The use of such a means for positioning under water requires a free space at the side of the vessel in order to arrange the buoys remote from the hull in order to achieve maximum precision. This prevents the inspection system from being used at the side of the hull directed towards the quay. The vessel must be moved in order to make it carry out a half-turn, in order to release the side of the hull which is initially close to the quay when it is desirable to carry out an inspection.
A known means for positioning in the air uses a system of the DGPS type, between markers on the ground and an antenna placed on the movable machine. The maximum precision of such a means for positioning in the air is of the order of 50 cm.
Another known means for positioning in the air involves an optical device which comprises a reference station which is fixed to the ground and an optical transmitter which is fixed to the robot. The reference station automatically points to the transmitter and transmits the three-dimensional position thereof with a precision which is within centimeters.
The first two above-mentioned positioning means do not provide the precision required to carry out an inspection of the hull of a vessel in an effective manner. The same defect of the hull must be prevented from being mapped several times as having different positions owing to a lack of precision of the position measurements of the machine. Otherwise, this would lead to the overestimation of the gravity of this defect and the implementation of significant maintenance operations of the hull which might require the immobilisation of the vessel. Conversely, a significant defect must be prevented from being underestimated owing to the lack of precision of the positioning means. In this manner, the desirable precision with respect to the positioning of such an inspection system is in the order of 10 cm.
Furthermore, it is desirable to be able to inspect the hull of the vessel over the entire surface thereof, that is to say, both below and above the water line, using a single machine. To this end, it is necessary to provide a positioning system which operates both in air and in water. It is also desirable for such a positioning system to allow the machine to be positioned with the same precision both below and above the water line.
In order to carry out instantaneous measurements of the position of the machine both in air and in water, and with the same precision, one possible solution is to provide the machine with an inertial unit. The integration of the movements of the machine during its movement from a reference point allows the instantaneous position of the machine to be determined. However, for the desired precision, the cost of a suitable inertial unit is high. Furthermore, the significant volume and the mass of an inertial unit are incompatible with the correct operation of the machine which must remain light and compact in order to adhere to and move on the hull of the vessel.